I did a LOT of work on my 919 this year. New chain/sprockets, 3 different sets of tires, clutch, front/rear brakes and I'm redoing the forks this week.
When I had the rear tire off, I noticed that the adjuster had enough room for a lock nut. A thin nut that would go on the thread where you see the threads as you adjust them. This and a thin washer could be torqued against the end of the swing arm once you find the right spot for the adjusters.
I didn't have the right nut for this, but either a single nut or double nut should do the trick. I think (haven't verified) that an open end wrench would still fit in there as this would be a small nut.
TBH, I don't know why they move, they don't seem to move much but several people have reported that it goes out of adjustment.
The nuts that I'm talking about would be about 1/2 the thickness of a standard nut and would need to have a good flat washer so as to not dig into the aluminum swing arm. If you look at the number of threads you have exposed, you should have enough to get a nut in there, maybe two thin ones to lock against each other. A hardened washer should keep the swing arm intact.